The Large UV/Optical/IR Surveyor (LUVOIR) is a concept for a highly capable, multi-wavelength space observatory with ambitious science goals. This mission would enable great leaps forward in a broad range of science, from the epoch of reionization, through galaxy formation and evolution, star and planet formation, to solar system remote sensing. LUVOIR also has the major goal of characterizing a wide range of exoplanets, including those that might be habitable - or even inhabited.
LUVOIR is one of four Decadal Survey Mission Concept Studies initiated in Jan 2016. The study will extend over three years and will be executed by the Goddard Space Flight Center, under the leadership of a Science and Technology Definition Team (STDT) drawn from the community.
UPDATE: On June 1, 2018, NASA HQ instructed the Decadal Mission studies (HabEx, LUVOIR, Lynx, and OST) to produce versions of their concepts that fit into the $3-5B cost box. LUVOIR was exempt from this instruction. The LUVOIR team will continue their efforts to develop observatory concepts that would provide broadly compelling scientific capability. LUVOIR's concepts, with attendant technology development plans, will be submitted to the Astro2020 Decadal Survey for consideration in the large mission class.
UPDATE: On June 14, 2018, NASA HQ withdrew the aforementioned June 1 memo and replaced it with new directions. The new memo acknowledges that all four studies are planning to design less costly second mission concepts, and notes that the LUVOIR-B architecture already under development has a roughly 50% size reduction compared to LUVOIR-A. HabEx, Lynx, and OST are given the goal of developing a second concept with an estimated cost less than about $5B. All four studies should submit all their mission concepts to the Astro2020 Decadal Survey, to provide a broad range of options for consideration.